Digital television display systems in the prior art produced line drawings by storing one video bit for every element of the picture. In many such prior art systems the raster assembly storage would have to store as many as one million video bits for a 1024 X 1024 raster matrix. The completed picture would then be transferred to a refresh store. One substantial drawback in such prior art displays is that any alteration in the displayed picture would require either the generation of a new picture or the moving of all one million bits to the raster assembly storage for modification and return. Thus to effect a single erasure of a single vector would require either the reassembly of the entire raster or the transfer of the entire one million bits out of the storage for alteration and replacement. In the event that two vectors cross one another, the process of erasing a first vector, after transfer back to the assembly store, would remove video bits common to both vectors, leaving the remaining vector with a gap separating the components on either side of the erased vector.
Some progress has been made in the prior art through the implementation of queue memories for the storage of digitally encoded video data. One example of such a prior art system discloses a video generator for data display which employs a threaded refresh buffer. The use of such a buffer permits a reduction in the size of the raster assembly storage over that of the prior art. However, this prior art image buffer must be large enough to accommodate the tallest character which is intended to be displayed. According to prior art teachings this would be at least eight raster lines which must be stored in the video image buffer. The prior art states that if a vector were to exceed the vertical height of such a video image buffer, it would have to be generated as separate segments. This, it is disclosed, would be accomplished by returning the contents of vector registers in the vector generator to the threaded list of the data buffer in order that the vector generator may continue at a later time in the scanning sequence. It is seen that the amount of processing necessary to access the next component of the vector in the next group of raster lines to be scanned, by accessing the threaded buffer itself, reduces the display capability of the system and increases its complexity.
What the art requires is an improved means of accessing subsequent components of vectors and other data stored in the system so as to enable higher rates for display.